The invention relates to an electroluminescent display screen having a transparent substrate, a surface of which is provided with a first, transparent electrode layer, with a layer of insulating material in which windows within which the first electrode layer is exposed are formed in accordance with a pattern to be displayed, with a layer of an organic electroluminescent material which is provided on the layer of insulating material and, within the windows in the layer of insulating material, on the first electrode layer, and with a second electrode layer which is provided on the layer of electroluminescent material. The invention also relates to a method of manufacturing such an electroluminescent display screen.
During operation of such a display screen, an electric current flows from the first electrode layer through the windows in the layer of insulating material to the second electrode layer. The layer of electroluminescent material then luminesces in accordance with the pattern formed in the layer of insulating material, which pattern is then visible through the transparent substrate. Patterns formed in the layer of insulating material can thus be displayed. A display screen of this type may be used, for example, in mobile telephones or watches, or in car or plane instrument panels. The pattern may be, for example, a word, a number or an icon.
A display screen of the type described in the opening paragraph is known from WO 98/03043, in which the first, transparent electrode layer is a layer of ITO (indium tin oxide) and the layer of insulating material is a layer of a positive photoresist. The photoresist is exposed in accordance with the pattern to be displayed and is subsequently developed, with the desired pattern being formed in the layer of insulating material. The layer of electroluminescent material, which consists of a customary double layer of an organic electroluminescent material, is deposited on the photoresist and, in the windows in the photoresist, on the first electrode layer. The second electrode layer consists of a layer of magnesium coated with a layer of silver. The insulating layer is also provided with a window which leaves open a bond pad of the first electrode layer for external contacting of the first electrode layer. This bond pad is located at the edge of the substrate, next to the pattern to be displayed. This bond pad is shielded by a mask during the deposition of the layer of electroluminescent material and the second layer of electrode material.
It has been found in practice that a pattern to be displayed may exhibit local differences in brightness during display. These differences particularly become manifest when the pattern to be displayed is, for example, a relatively large word or icon.